I. Field of the Invention
This invention relates generally to the field of nanoparticle electrocatalysts. In particular, the present invention relates to the controlled deposition of a smooth and conformal catalytically active surface layer on high-surface-area carbon nanostructures. This invention further relates to the use of these coated nanostructures as electrocatalysts in energy conversion devices such as fuel cells, batteries, capacitors, and supercapacitors.
II. Background of the Related Art
The emerging global energy crisis has resulted in a renewed interest in the development of new and improved energy conversion devices. The production of useful energy generally requires the transformation of an energy source from one state into another state which is capable of being used by the consumer. Improvements in conversion efficiency enable production of larger quantities of useful energy from a given quantity available from an energy source. Some examples of present-day energy conversion devices include capacitors, batteries, supercapacitors, and fuel cells. Each of these will, along with some of their associated problems, be briefly discussed below.
In its simplest form, a capacitor is an energy storage device comprised of two conducting plates separated by an insulating layer. When a voltage is applied to the plates, positive and negative charges are induced on opposite surfaces and an electric field is generated. The ability of a capacitor to store electrical charge is defined as its capacitance which is directly proportional to the polarizability of the insulating layer and the surface area of the plates, but is inversely proportional to the separation between the plates. Thus, the larger the plate surface area, the greater the polarizability of the insulating medium; and, the smaller the plate separation, the greater the resulting capacitance.
Batteries, on the other hand, generally produce electrical energy by the oxidation and reduction of electrochemical reagents within the battery. In this case the energy storage and conversion process is Faradaic since electron transfer between the electrodes occurs. Charge storage in capacitors is generally non-Faradaic since the storage of electrical charge is fully electrostatic with no electron transfer occurring across the electrode interface. While batteries are capable of attaining high energy densities over a wide range of voltages, they cannot attain high power densities and can only undergo a limited number of recharge cycles. Capacitors can provide high energy transfer rates with a nearly unlimited number of recharge cycles, but have limited charge storage capabilities.
Advances in energy storage devices eventually led to the development of the electric double-layer capacitor which is also known as an electrochemical capacitor or supercapacitor. A supercapacitor is an electrochemical energy storage device which combines the high energy storage capabilities of a battery with the high power and nearly unlimited recharging cycles attainable with a capacitor. A comprehensive review of the development and operation of supercapacitors is provided by B. E. Conway in “Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications,” Kluwer Academic/Plenum Publishers, NY (2003) the entire contents of which is incorporated by reference as if fully set forth in this specification.
A supercapacitor is generally comprised of opposing porous, yet polarizable electrodes which are interspersed with an electrolyte and separated by an exceedingly thin, yet insulating and porous separator layer. The electrodes themselves are typically comprised of a porous material having a very large surface area. This assembly is situated between two opposing current collectors, each of which is in contact with an outer surface of an electrode. The exceedingly thin separator layer combined with the large surface area of the electrodes yields a device with an extraordinarily high capacitance.
Nanoporous electrode materials such as graphite, carbon fibers, charcoal, vitreous carbon, carbon aerogels, and activated carbon have previously been employed as supercapacitor electrodes. Factors which affect the charge storage efficiency of such carbon-containing electrodes include the availability of surface area for the accumulation of charge, electrolyte accessibility to intrapore surfaces, electrical conductivity within porous matrices, as well as the chemical stability and electrical conductivity of the electrode itself. Activated carbon is commonly employed as the electrode material due to its relatively large specific surface area which is on the order of 1000 to 2000 m2/g. However, its small pore size (typically a few nm in diameter) makes it difficult for ions in the electrolyte to access intrapore surfaces. Furthermore, the use of insulating polymeric binders to fabricate the electrodes is detrimental to performance since it increases the resistance of the electrode.
Some of the problems associated with activated carbon may be circumvented by using carbon nanotubes as the electrode material. Carbon nanotubes are nanometer-scale cylindrical structures comprised entirely of sp2 bonded carbon atoms. Although the specific surface area of carbon nanotubes may be considerably lower than that of activated carbon or carbon fiber, electrodes with a higher capacitance per unit surface area and lower internal resistance can be obtained. This is due primarily to the larger pore structure of carbon nanotube aggregates which permit greater accessibility to the available surface area. However, access to inner wall surfaces of nanotubes is inhibited by the small diameter of the tube ends and its proportionally larger length.
A still higher capacitance may be obtained using carbon nanohorns which have a structure analogous to nanotubes, but with one end of the cylindrical tube closed and the other open, resulting in a horn-like shape. Since carbon nanohorns have a more open structure, both the internal and outer surfaces of carbon may be made accessible to adsorbates. Consequently carbon nanohorns generally possess a higher specific surface area than carbon nanotubes and an average pore size (on the order of tens of nm) which is larger than both carbon nanotubes and activated carbon or carbon fibers.
From among available metal electrocatalysts, ruthenium exhibits the most potential for improving the storage capability because of its multivalent states which permit greater charge storage through an oxidation reaction wherein Ru→Ru4+. Furthermore, Ru remains adsorbed on the surface even after undergoing a change in oxidation state. The utilization of Ru is, however, inhibited by the high cost and scarcity of Ru as well as the toxicity of its oxides. Controlled deposition of smooth, conformal thin films of Ru in the submonolayer to multilayer thickness range is also difficult to achieve. This is primarily due to the tendency of Ru to form films having a high surface roughness with granular nanoparticles dispersed across its surface.
While batteries, capacitors, and supercapacitors operate by releasing a finite amount of stored energy, a fuel cell has the advantage of being able to operate continuously. A fuel cell is itself an electrochemical device which operates by converting the chemical energy of a fuel and an oxidant into electrical energy. A standard fuel cell is comprised of an anode and cathode separated by a conducting electrolyte which electrically insulates the electrodes yet permits the flow of ions between them. The fuel cell operates by separating electrons and ions from the fuel at the anode and transporting the electrons through an external circuit to the cathode. The ions are concurrently transported through the electrolyte to the cathode where the oxidant is combined with the ions and electrons to form a waste product. An electrical circuit is completed by the concomitant flow of ions from the anode to cathode via the conducting electrolyte and the flow of electrons from the anode to the cathode via the external circuit.
The science and technology of fuel cells has received considerable attention, being the subject of numerous books and journal articles including, for example, “Fuel Cells and Their Applications,” by K. Kordesch and G. Simader, New York, N.Y.: VCH Publishers, Inc. (2001). Although there are various types of fuels and oxidants which may be used, the most significant is the H2—O2 system. In a hydrogen-oxygen fuel cell, hydrogen (H2) is supplied to the anode as the fuel where it dissociates into H+ ions and provides electrons to the external circuit. Oxygen (O2) supplied to the cathode undergoes a reduction reaction in which O2 combines with electrons from the external circuit and ions in the electrolyte to form H2O as a byproduct. The overall reaction pathways leading to oxidation at the anode and reduction at the cathode are strongly dependent on the materials used as the electrodes and the type of electrolyte.
Under standard operating conditions the H2 and O2 oxidation/reduction reactions proceed very slowly, if at all, requiring elevated temperatures and/or high electrode potentials to proceed. Reaction kinetics at the electrodes may be accelerated by the use of metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related noble metal alloys. Electrodes formed of these materials function as electrocatalysts since they accelerate electrochemical reactions at electrode surfaces yet are not themselves consumed by the overall reaction. Further improvements have been attained by incorporating noble metal-containing particles or structures with reduced dimensions. A reduction to nanoscale dimensions yields a significant increase in the surface-to-volume ratio, thereby producing a concomitant increase in the surface area available for reaction. Despite the performance improvements attainable with nanoscale electrocatalysts, successful commercialization of fuel cells requires still further increases in performance and cost efficiency.
Pt has been shown to be one of the best electrocatalysts, but its successful implementation in commercially available fuel cells is hindered by its extremely high cost, susceptibility to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and the relatively slow kinetics of O2 reduction at the cathode. A variety of approaches have been employed in attempting to solve these problems. An example is U.S. Pat. No. 6,232,264 to Lukehart, et al. which discloses polymetallic nanoparticles such as platinum-palladium alloy nanoparticles for use as fuel cell electrocatalysts. Another example is U.S. Pat. No. 6,670,301 to Adzic, et al. which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported on carbon substrates. These approaches have resulted in electrocatalysts with reduced Pt loading and a higher tolerance for CO poisoning. Both of the aforementioned patents are incorporated by reference as if fully set forth in this specification.
Attempts to accelerate the oxygen reduction reaction (ORR) on Pt while simultaneously reducing Pt loading have been met with limited success. Recent approaches have utilized high surface area Pt or Pd nanoparticles supported by nanostructured carbon (Pt/C or Pd/C) as described, for example, in U.S. Pat. No. 6,815,391 to Xing, et al., which is incorporated by reference as if fully set forth in this specification. However, as an oxygen reduction catalyst, bulk Pt is still several times more active than Pt/C and Pd/C nanoparticle electrocatalysts. Another approach involves the use of Pt-encapsulated core-shell or alloy nanoparticles as described, for example, in U.S. Patent Publ. No. 2007/0031722 to Adzic, et al., which is incorporated by reference as if fully set forth in this specification. The quantity of noble metal required was reduced even further by using a core-shell nanoparticle with a noble metal shell, but a non-noble metal core.
Despite the continued improvement attained with modern nanoparticle electrocatalysts, successful implementation in commercial energy conversion devices requires still further increases in the catalytic activity while simultaneously improving long-term stability and reducing the amount of costly precious metals required.